This application is based on Japanese Patent Application No. 2003-415225 filed on Dec. 12, 2003, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a solid-state image-sensing device having a plurality of pixels, and more particularly to a solid-state image-sensing device that can vary the sensitivity and resolution thereof to suite a given image-sensing subject or operation.
2. Description of Related Art
Conventionally, a solid-state image-sensing element that performs linear conversion operation to yield a signal linearly proportional to the amount of incident light has a narrow, specifically two-digit, dynamic range. Thus, when this type of solid-state image-sensing element is used to sense a subject having a wide-ranging brightness distribution, it does not output brightness information outside the dynamic range thereof. On the other hand, a different type of solid-state image-sensing element is conventionally known that performs logarithmic conversion operation to yield a signal logarithmically proportional to the amount of incident light. This type of solid-state image-sensing element has a wide, specifically five- to six-digit, dynamic range, and therefore, when it is used to sense a subject having a considerably wide brightness distribution, it can convert all the brightness information within the brightness distribution into an electrical signal and output it. With this type of solid-state image-sensing element, however, the sensible range is so wide relative to the brightness distribution of the subject that, in a low-brightness or high-brightness region within the sensible range, there is left a region where no brightness data is present. To overcome this inconvenience, the applicant of the present invention has ever proposed a solid-state image-sensing element of which the operation can be switched between the linear and logarithmic conversion operation mentioned above.
In a solid-state image-sensing device incorporating such a solid-state image-sensing element, to achieve a high frame rate, the solid-state image-sensing element, if it has a large number of pixels, needs to be operated by being fed with a high-frequency pulse signal. Driving a solid-state image-sensing element with a high-frequency pulse signal, however, causes it to consume high power. In addition, since the operation duration per cycle becomes short, the exposure duration becomes accordingly short. This makes it impossible to achieve sufficient exposure in each pixel, and thus makes the level of the signal obtained from the solid-state image-sensing element low. As a result, an image sensed with such a solid-state image-sensing device has low contrast. To avoid this, conventionally, scanning is performed on a “thinned-out” basis, specifically by making only part of the pixels in each row operate while keeping the others unoperating. This helps lower the frequency of the drive pulse signal.
In another conventionally proposed type of solid-state image-sensing device, between every two vertically adjacent pixels, the photoelectric conversion portions formed therein, each composed of a photodiode and a capacitor, can be coupled together to achieve high-sensitivity image sensing. In this solid-state image-sensing device, a MOS transistor serving as an output stage is provided for every two rows, and each pixel is provided with a switch that connects and disconnects such a MOS transistor to and from the node between photodiode and capacitor of the pixel.
These conventional techniques, however, have the following disadvantages. Thinned-out scanning keeps some pixels unoperating, and thus uses only a smaller number of pixels than are actually provided. This makes the solid-state image-sensing element incorporated in the solid-state image-sensing device equivalent to one having a lower aperture ratio, lowering the sensitivity thereof. On the other hand, in the solid-state image-sensing device described in the previous paragraph, pixel coupling is performed only vertically. This makes it impossible to switch sensitivities flexibly with respect to both the vertical and horizontal resolutions. Moreover, since the MOS transistors serving as the output stages are provided only one for every two rows, and thus each output stage is shared between two vertically adjacent pixels, reading electrical signals from all the pixels requires complicated control.